Data transfer networks include network elements such as, for example, routers, switches, and terminal devices which communicate with each other via data transfer links between the network elements. In many data transfer networks, there is a need to achieve phase or at least frequency synchronization between clock signals prevailing at various network elements of a data transfer network. The network elements can be configured to constitute master-slave pairs in order to distribute phase or at least frequency information within a data transfer network. Each slave network element controls its clock signal generator so that a reference clock signal prevailing at the corresponding master network element is regenerated in the slave network element on the basis of timing messages transferred from the master network element to the slave network element. The timing messages can be time-stamps contained by protocol data units “PDU” that can be, for example, data packets or data frames. Each time-stamp indicates the instantaneous time value at the transmission moment of the respective protocol data unit containing the time-stamp under consideration, where the time value is based on the reference clock signal available at the master network element. The timing messages can be timing packets that are transmitted so that the time interval between transmission moments of two successive timing packets is constant, or otherwise known, when being measured with the reference clock signal available at the master network element. It is also possible that one or more time-stamps indicating the transmission moments of one or more timing messages are transferred in one or more data packets transmitted after the one or more timing messages. It is also possible that the timing information indicating the phase or at least frequency difference between the reference clock signal and the regenerated clock signal is derived not only from timing messages, e.g. time stamps or timing packets, transferred from the master network element to the slave network element but also from timing messages transferred in the opposite direction from the slave network element to the master network element. The use of the timing messages transferred from the slave network element to the master network element requires, however, an additional data transfer from the master network element to the slave network element.
The timing messages, e.g. the time stamps or the timing packets, may be received at a slave network element from several master network elements, i.e. from several timing sources, each of which having an own reference clock signal. US2008/0049743 teaches to make a selection between different timing sources on the basis of Allan Variance estimators which are indicators of frequency stability. US2008/0049743 does not, however, teach any technical solution for improving the utilization of the information content of a single flow of timing messages.